This invention relates to electronic circuitry for control of electrical power and particularly to digital logic circuitry for controlling the mode of operation of a solid state relay.
In electrical power systems there has been increasing interest in the use of solid state relays for controlled energization of a load from a power supply. For DC circuits, this usually takes the form of some kind of transistorized switching circuit such as is shown in Baker U.S. Pat. No. 3,898,552, issued Aug. 5, 1975.
In various applications, different modes of operation are required from a solid state relay. While the same basic power switching circuit may be employed generally in each of these modes of operation, they each require a different sort of control circuit in order to make the power switch operate in the required manner.
The functions desired of a solid state relay are directly analogous to the functions of electromechanical relays. The required functions include those referred to as normally open (NO), normally closed (NC), and latched. In normally open operation, the solid state relay (SSR) is to be OFF or to present an open circuit between the source and the load unless and until a control signal is applied to the power switch to close it and complete the circuit between the source and the load. In the normally open mode, the power switch will remain closed only so long as such a control signal is applied to it. In normally closed operation, the converse is to occur with the switch ON and the load circuit closed except when a control signal of a certain type is present that causes and maintains interruption of the load circuit. In both the normally open and normally closed modes of operation, a change of state of the relay occurs only as a result of a change in the applied control signal. In the third mode of operation, the latched mode, it is the case that the state of the relay can be changed by application of a control signal and will remain in the same changed state when the control signal is removed until a further control signal is applied to change the state again.
All of the types of operation referred to in the preceding paragraph relate to relays that may be called single pole, single throw (SPST) relays because the operation of the relay is to connect or disconnect between a single line or power pole and a single load. There are, however, systems that require operation between a plurality of lines and a single load or a single line and a plurality of loads with some interrelation of the individual relay functions such that one is to remain in or out of phase with the other. For this purpose, it is necessary that the relays be adapted for associating different control circuits in master-slave relationship.
Because of the number and variety of functions desired of an SSR, it has been previously necessary to make and use different specific control circuits for different ones of the required SSR functions with the natural consequence of poor economics as compared with a single circuit that could serve multiple purposes.